We're occasionally asked to implement dynamic port forwarding, where
PuTTY acts as a proxy for connection requests (using a protocol such
as SOCKS) and creates tunnels on the fly. This is akin to
OpenSSH's-D option.
From the 3.5p1 ssh(1) man page:

-D port
Specifies a local ``dynamic'' application-level port forwarding.
This works by allocating a socket to listen to port on the local
side, and whenever a connection is made to this port, the connec-
tion is forwarded over the secure channel, and the application
protocol is then used to determine where to connect to from the
remote machine. Currently the SOCKS4 protocol is supported, and
ssh will act as a SOCKS4 server. Only root can forward privi-
leged ports. Dynamic port forwardings can also be specified in
the configuration file.